• 析構器只適用于類類型，當一個類的實例被釋放之前，析構器會被立即調用。析構器用關鍵字deinit來標示。

析構過程原理（How Deinitialization Works）

• Swift 會自動釋放不再需要的實例以釋放資源。

• 在類的定義中，每個類最多只能有一個析構器，而且析構器不帶任何參數。

• 析構器是在實例釋放發生前被自動調用。

• 不能主動調用析構器。

• 子類繼承了父類的析構器，并且在子類析構器實現的最后，父類的析構器會被自動調用。

• 因為直到實例的析構器被調用后，實例才會被釋放，所以析構器可以訪問實例的所有屬性，并且可以根據那些屬性可以修改它的行為（比如查找一個需要被關閉的文件）。

class Bank {
    static var coinsInBank = 10_000
    static func vendCoins(numberOfCoinsRequested: Int) -> Int {
        let numberOfCoinsToVend = min(numberOfCoinsRequested, coinsInBank)
        coinsInBank -= numberOfCoinsToVend
        return numberOfCoinsToVend
    }
    static func receiveCoins(coins: Int) {
        coinsInBank += coins
    }
}

class Player {
    var coinsInPurse: Int
    init(coins: Int) {
        coinsInPurse = Bank.vendCoins(coins)
    }
    func winCoins(coins: Int) {
        coinsInPurse += Bank.vendCoins(coins)
    }
    deinit {
        Bank.receiveCoins(coinsInPurse)
    }
}

var playerOne: Player? = Player(coins: 100)
print("A new player has joined the game with \(playerOne!.coinsInPurse) coins")
// 打印 "A new player has joined the game with 100 coins"
print("There are now \(Bank.coinsInBank) coins left in the bank")
// 打印 "There are now 9900 coins left in the bank"

playerOne!.winCoins(2_000)
print("PlayerOne won 2000 coins & now has \(playerOne!.coinsInPurse) coins")
// 輸出 "PlayerOne won 2000 coins & now has 2100 coins"
print("The bank now only has \(Bank.coinsInBank) coins left")
// 輸出 "The bank now only has 7900 coins left"

playerOne = nil
print("PlayerOne has left the game")
// 打印 "PlayerOne has left the game"
print("The bank now has \(Bank.coinsInBank) coins")
// 打印 "The bank now has 10000 coins"